Fiber reinforced plastics (FRP) reinforced by a group of continuous reinforcing fibers are often used as materials of structures requiring lightweight and mechanical properties for such applications as transport apparatuses including aircraft, automobiles, two wheelers and bicycles, sports goods including tennis rackets, golf club shafts and fishing rods, architectural structures including earthquake-resisting reinforcing materials, etc.
As a lighter weight structure having sufficient mechanical properties, a sandwich structure in which one each FRP component is disposed on the skin members of a lightweight core component is known. For reducing the weight of a structure, a core component lighter in weight is selectively used, and a balsa core, honeycomb core, urethane foam core, etc. are frequently used as core components. Further, such sandwich structures are designed to satisfy practically required mechanical properties and are widely used as secondary structural materials of aircraft and also as automobile members, architectural members, panel members, etc.
Patent Document 1 discloses a foamed core component in which resin is foamed and a core component in which monofilaments of discontinuous reinforcing fiber intersect one another via thermoplastic resin as a core component of a sandwich structure. Patent Document 2 discloses a lightweight composite material having a layered structure composed of a fiber-reinforced resin layer of a fiber for reinforcement and a porous fiber layer produced by binding short fibers for reinforcement dispersed in random directions, with a binder at their intersections.
However, in case of these sandwich structures or composite materials, it is difficult to satisfy both stiffness and lightweight, and there is a limit for mass-producing a thin-walled complicatedly shaped article. Since the core component of the sandwich structure disclosed in Patent Document 1 is produced using particles of a thermoplastic resin and a discontinuous reinforcing fiber, it was difficult to well balance the stiffness and the lightweight of the core component even though monofilaments of the discontinuous reinforcing fiber intersect one another via the thermoplastic resin. The lightweight composite material disclosed in Patent Document 2 is poor in moldability of the core component because of the use of carbide obtained by calcining a thermosetting resin or the like as the resin for forming a core component.
On the other hand, the applications of FRP include housings of electric and electronic apparatuses such as personal computers, office automation apparatuses, audiovisual apparatuses, cell phones, telephone sets, facsimiles, household electric appliances and toys. These applications require mass productivity, moldability, productivity and economic efficiency, and in recent years, additionally, thinness and lightweight. To meet these requirements, magnesium alloys excellent in thinness and stiffness may be used, but since metallic materials have large specific gravities, they are not quite satisfactory in view of lightweight. It is anticipated that electronic apparatuses such as notebook-type personal computers, telephone sets and information terminals will be increasingly made portable and that aged users will further increase in future. Therefore, it is highly demanded that these electronic apparatus housings are further reduced in weight.